Solutions containing caustic alkali are industrially discharged from various production processes, treating processes or processing processes. Examples of such solutions include reaction waste solutions from various chemical reaction processes, waste solutions from treating metals with alkali, waste solutions from regenerating ion exchange resins, alkali-treating waste solutions from petroleum refining processes, alkali-treating waste solutions from nuclear energy facilities and the like. It is industrially important to recover caustic alkali from these waste solutions in view of both the economics of the processes and prevention of pollution.
For these reasons, various methods for recovering or detoxicating caustic alkali by treating these waste solutions have heretofore been tried. Most of these alkali-containing waste solutions are aqueous solutions of a relatively low concentration and contain many other inorganic or organic coexisting substances. Therefore, the solutions are often discharged after detoxicating by neutralization, etc. without a recovering treatment, for technical or economic reasons.
Electrolytic methods using a cation exchange membrane are known as representative methods for effectively recovering caustic alkali from these waste solutions. For example, a method for treating an alkaline waste water which comprises separating and recovering an alkali from the alkaline waste water by electrodialysis using a cation exchange membrane and discharging the waste water as a neutralized water is described in Published Unexamined Japanese Patent Application 16859/1977.
However, such electrolytic methods are disadvantageous since material which is highly durable in an oxygen generating reaction is required as an electrode, particularly as an anode, and expensive noble metal or easily exhaustive graphite, which has various disadvantages in production or operation, must be used.
Accordingly, it is desired to develop a technically and economically excellent electrolytic technology which can be industrially operable.
Iron, nickel and their base alloys such as stainless steel, etc. are inexpensive, easy to process and, thus, have heretofore been used as an electrode for electrolysis of a caustic alkali aqueous solution in water electrolysis, etc. However, these materials can only be used in an aqueous solution having a high caustic alkali concentration and at a relatively high temperature. These materials cannot be used as an electrode when electrolyzing a low concentration caustic alkali aqueous solution because deactivation occurs by formation of an oxide on the surface of the electrode due to considerable oxidation of the anode by elevation of the electrolytic voltage or because dissolution of the surface of the anode occurs at a low caustic alkali concentration of about 10 wt% or less, particularly 5 wt% or less.
Furthermore, in the case of electrolysis of a waste solution containing various organic substances and heavy metals, these impurities attach and precipitate onto the ion exchange membrane, electrode or pipes, and make the electrolysis difficult.